IMO, a lot of the "heavy" comes from attitude..... AKA having the balls to play with low gain, allowing the listeners to really hear the strings moving, the tubes working, the woods resonating..... basically, exposing the music and not hiding behind hot amps. Gain is nice... there is a balance somewhere. But error on the low side. It might not be as fun to play.... you'll have to work a little harder, but it can sound fantastic. And if your stage presence sucks, and lyrics suck, and everything sucks but the riffs... well, the last thing you want to do is make them mush behind too much gain.
Nothing says your riffs are heavy and your balls are big like full exposure.